This invention relates in general to the manufacture of paper, and more specifically to an improved process for removing ink from a slurry of pulped waste paper using a foam flotation technique to produce new paper.
Economies of manufacture and the conservation of natural resources has generated interest in the manufacture of paper from recycled stock, where paper previously printed is pulped to create a slurry that is fed to a paper-making machine. Printing inks must be removed from the slurry to produce new paper of the appropriate brightness.
Various techniques for removing the printing inks are known, but the present invention relates to the froth or foam flotation process where defibered paper is mixed with preheated water and chemicals including alkalis, collectors and foamers. This treated raw slurry is directed to a tank where it is aerated with microscopic bubbles stabilized by a flotation agent or agents. As the bubbles rise in the tank through the slurry the ink pigments adhere to the bubbles and are carried to the surface of the tank where they form a foam or froth. The foam is skimmed away or otherwise removed, and the deinked slurry is treated further, e.g. by processing in a stockwasher described in applicant's U.S. Pat. No. 4,151,093. Dirty outlet water from the stockwasher can be cleaned in a clarifier of the type described in applicant's U.S. Pat. Nos. 4,022,696; 4,377,485; or 4,626,345 and used in the paper-making process or returned to a water supply such as a river or lake.
U.S. Pat. No. 4,157,952 to applicant discloses a prior art foam flotation deinker. A slurry of 1% pulped waste paper is introduced into a cylindrical tank by a nozzle positioned in the tank, under water, and near the upper, outer edge of the tank. The nozzle is pointed downwardly at 30.degree. to 60.degree. and ejects a flow tangent to the outer tank wall to set up a swirling flow within the tank. The nozzle includes internal restrictions and an air line located after one set of restriction to draw air into the slurry. The flow into the nozzle is pressurized at 10-20 psi. A baffle extends generally radially across the upper surface of the tank. After almost one rotation within the tank, the swirling floated sludge strikes the baffle. A vacuum foam inlet at the baffle removes the accumulated foam and directs it to a vacuum tank where the air bubbles burst and the ink residue collects, and is discharged.
While this '952 deinker works well as compared to the prior art foam flotation apparatus described in this '952 patent, it is presently the component that limits the speed of the pulp treatment line feeding the paper-making machine. If the throughput rate is increased, the new paper quality will be adversely affected. The difficulty of meeting the necessary production capacity conventional equipment has, at least in part, caused some paper mills to close their operations.
The straightforward expedient of using a larger diameter tank does not solve the problem. While the capacity of a foam flotation de-inker increases with the available surface area, current production requirements require a prohibitively large tank in terms of size, weight, capital cost, and energy usage to operate it. Larger tanks are also, in general, associated with a longer retention time which is not conducive to efficient operation.
It is therefore a principal object of the present invention to provide an improved foam flotation apparatus and method with a larger capacity than heretofore possible.
A further object is to provide an improved apparatus and method which has a low weight and does not require special reinforcement to support it.
Yet another object is to provide a method and apparatus with the foregoing advantages which is also efficient both in terms of deinking and power consumption.
A further object is to provide an apparatus and process for deinking pulped waste paper which is modular in construction to tailor capacity and power consumption to a specific application.
A still further object is to provide the foregoing advantages using comparatively uncomplicated apparatus and techniques that can be implemented at a favorable capital cost.
Another object of the invention is to provide a deinker apparatus and method with the foregoing advantages which can treat pulped waste paper at a rate and efficiency sufficient to keep up with other modern pulp processing equipment and to protect the environment from the discharge of contaminants.